Arrangements previously used for the above mentioned purpose, e.g. as disclosed in FI Patent 77276, include devices wherein pulp in a bleaching stage is supplied either subsequent to a pressure screen or directly to a vortex cleaning plant in which the heavier impurities still in the pulp, such as shives, large fiber flocs and the like, are separated therefrom. As is known, a vortex cleaning plant comprises dozens of vortex cleaners, i.e. hydrocyclones, the cleaning effect of which is based on the recirculation of the material to be cleaned along the cylindrical or conical wall of the cleaner at a high velocity. The coarse material comprising the reject fraction accumulates due to the centrifugal force on the surface of the cyclone rapidly wearing it out, thereby firstly diminishing the cleaning efficiency of the cleaner when the rotational speed of the material rapidly decreases due to additional friction eventually resulting in one or more holes in the wall of the vortex cleaner through which the material to be cleaned will escape to the floor of the screening space. Further, a vortex cleaning plant requires a rather large space mainly due to the large number of individual cyclones. The use of cyclones also has a third disadvantage, namely a significant increase in water circulation and effluent loading. To operate efficiently the consistency of the fiber suspension to be cleaned with vortex cleaners must be tenths of one per cent, so that the acceptable fibrous material may be effectively separated from knot particles and shives.
Yet another disadvantage of a vortex cleaning plant is that the consistency of the pulp flowing to the inlet box of the washers most frequently used in a bleaching plant must be approximately 1.2%, at least about 1.0%. Similarly, the drying apparatus operating as a treatment stage subsequent to bleaching requires a supply consistency of the same about 1.0-1.2%. This requires that the pulp must be thickened from about 0.5% to the consistency of about 1.0-1.2%.
The aforementioned disadvantages of the apparatus in accordance with the prior art may be eliminated by replacing the vortex cleaning plant, for example, with narrow-slot screening, whereby one single screener performs the work of tens of vortex cleaners. At the same time the thickener previously required subsequent to the vortex cleaning plant has become superfluous. Savings in the space is thus self-evident as well as the decrease in liquid loading, because the suspension to be treated does not have to be diluted to the level required by the vortex cleaning plant, rather, it may be left at the consistency level of the inlet box of a washer or a drying apparatus. Consequently, the demand on the dilution liquid is significantly diminished. It has also been suggested that the screening itself were carried out at a medium consistency with a fluidizing screener. For example, FI patent application 781789 corresponding to CA patent 1,102,604, and published SE patent application 465 781 relate to such arrangements. Said screening method has at least not at the present been accepted even for coarse screening at mill scale, so it is very unlikely that fine screening will ever be carried out at medium consistency. Thus at least for the present everything is based on the fact that pulp must be considerably diluted between the bleaching stages or thereafter prior to the screening. The characterizing features of the method and arrangement in accordance with the present invention become apparent in the enclosed claims.